The invention relates to the field of power electronics. It is based on a method for operating a transformer from a drivable voltage source, and on an apparatus for carrying out the method as claimed in the precharacterizing clause of the independent claims.
Transformers in conjunction with drivable voltage sources, in particular drivable voltage sources in the form of voltage converters, are nowadays used in a range of power-electronic circuits. One such voltage converter is disclosed in U.S. Pat. No. 5,309,346 and is normally connected via the transformer to an electrical AC voltage supply network and feeds and/or draws active power and/or a reactive component, in particular for stabilization of the network voltage of the electrical AC voltage supply network. The voltage converter is in this case driven by means of drive signals from a drive circuit that is normally provided. Furthermore, an electrical load which is connected to the electrical AC voltage supply network is supplied with electrical power via the transformer from the electrical AC voltage supply network and/or from the voltage converter.
In the case of a serious fault in the electrical AC voltage supply network, in particular resulting from a short circuit of the electrical AC voltage supply network, this network is disconnected from the electrical load by means of an isolating switch. The voltage converter must then provide the voltage required by the electrical load and the corresponding current via the transformer. The output current to be provided by the voltage converter is governed by the magnetization current of the transformer and by the load current for the electrical load. In the fault situation described above, the magnetization current will increase within a few microseconds after the occurrence of the short circuit, as a result of which the transformer is magnetized and may be driven into saturation. As a result of the high magnetization current, the output current to be provided by the voltage converter, that is to say the transformer current, can rise above the maximum permissible value for the voltage converter, as a result of which a protective apparatus which is normally provided, in particular an isolating switch, then blocks the voltage converter, in order to protect its power semiconductor switches, when the maximum permissible value is exceeded. However, this does not ensure that the electrical load is adequately supplied with power within a small number of time periods of the network voltage.
One known possible way to set the converter current below the maximum permissible value for the fault situation described above, and thus to ensure that the electrical load is supplied with electrical power, is to derate the transformer, in particular the iron core of the main inductance. In consequence, the magnetization current of the transformer can be effectively limited during the fault situation explained above, so that the output current can be kept below the maximum permissible value.
The derating of the transformer as described above results in the problem that this is accompanied by an increased space requirement and an increased use of material, so that the derating results in considerable costs.
The object of the invention is therefore to specify a method for operating a transformer from a drivable voltage source, which allows an electrical load which is fed from an electrical AC voltage supply network to be supplied by means of the voltage source via the transformer within a short time when a fault occurs in the electrical AC voltage supply network. Furthermore, an apparatus is to be specified, by means of which the method is carried out in a particularly simple manner. This object is achieved by the features of claims 1 and 11. Advantageous developments of the invention are specified in the dependent claims.
In the method according to the invention for operating a transformer from a drivable voltage source, the voltage source is driven such that it feeds active power and/or a reactive component via the transformer into an electrical AC voltage supply network, with an output current from the voltage source being monitored for a maximum permissible value. According to the invention, a transformer voltage which is applied to the transformer and the magentic flux of the transformer are determined continuously. If the maximum permissible value of the output current is exceeded, that is to say if a fault occurs in the electrical AC voltage supply network, in particular in the event of a short circuit occurring, the voltage source is driven such that the voltage source is disconnected from the transformer, with the power semiconductor switches in the voltage source preferably being blocked. In the fault situation mentioned above, this advantageously prevents the voltage source from being loaded with an output current which is to be supplied but which, as a result of the fault situation, is greater than the maximum permissible value, and prevents components in the voltage source, in particular power semiconductor switches, from being damaged or even destroyed. Furthermore, there is no need for any isolating switch, as is normally provided, for the disconnection of the voltage source.
Furthermore, according to the invention, in response to a start signal, an output voltage from the voltage source is formed as a function of a transformer voltage, which was determined most recently before the maximum permissible value of the output current was exceeded, and as a function of a first magnetic flux, which was determined most recently before the maximum permissible value of the output current was exceeded, and of a second magnetic flux determined at the time of the start signal. The voltage source is then driven such that the output voltage is applied to the transformer for an adjustable time period after an adjustable switching time. The output voltage which is formed represents a start value, so that the transformer, which is partially demagnetized or is essentially completely demagnetized in the event of a fault, can be brought to its original magnetization, that is to say to its magnetic flux as known before the fault situation. The application of the output voltage which is formed to the transformer after the adjustable switching time for the adjustable time period results in the magnetic flux building up to its value as known before the fault situation, that is to say up to its magnetization as known before the fault situation, as quickly as possible in an advantageous manner, without the maximum permissible value of the output current from the voltage source being exceeded in the process. This ensures that the electrical load can be supplied by the voltage source via the transformer within a short time when a fault occurs, as described above, in the electrical AC voltage supply network. The method according to the invention can be used in general in an advantageous manner for partially demagnetized or essentially completely demagnetized transformers, and is thus not restricted to the fault situation described above. Furthermore, the method according to the invention allows the transformer to be connected by means of the voltage source as quickly as possible and without any transients.
The apparatus according to the invention for carrying out the method for operating a transformer from a drivable voltage source has a drive device which is used for driving the voltage source, with the voltage source producing an output voltage, in accordance with the drive, for feeding active power and/or a reactive component via the transformer into the electrical AC voltage supply network. Furthermore, according to the invention, a first monitoring device is provided for monitoring the output current from the voltage source for a maximum permissible value and is connected on the input side to the drive device, with the drive device being driven, if the maximum permissible value of the output current from the voltage source is exceeded, such that the voltage source is disconnected from the transformer, preferably by blocking the power semiconductor switches in the voltage source. Furthermore, according to the invention, first means are provided for continuously determining the transformer voltage which is applied to the transformer, as well as a reference signal generator, which has second means for continuously determining the magnetic flux of the transformer and which is connected on the input side to the first means and on the output side to the drive device. The reference signal generator is advantageously designed such that, in response to the start signal which is supplied to the reference signal generator, an output voltage reference signal which corresponds to the output voltage is formed as a function of the transformer voltage which was determined most recently before the maximum permissible value of the output current was exceeded, and as a function of the first magnetic flux and of the second magnetic flux. Furthermore, the reference signal generator has an enable switching device, to which the output voltage reference signal, an adjustable switching time and an adjustable time period are supplied in order to enable the output voltage reference signal at the output of the reference signal generator. The apparatus according to the invention thus makes it possible for the method according to the invention for operation of a transformer from a drivable voltage source to be carried out in a particularly simple manner, in which case, furthermore, the apparatus can be produced very easily and at low cost, since the circuit complexity can be kept extremely low and, furthermore, only a small number of components are required to construct it.